Bidirectional DC/DC converter

ABSTRACT

A power conversion device, which includes an insulation type full bridge converter and can switch a power transmission direction at a high speed, is provided. A DC/DC converter (10) constitutes a power conversion device, which operates as a first type converter that converts a voltage within a first range applied to a first input/output terminal pair into a voltage within a second range and outputs the voltage from a second input/output terminal pair or a second type converter that converts a voltage within the second range applied to the second input/output terminal pair into a voltage within the first range and outputs the voltage from the first input/output terminal pair, as a device that performs a specific state transition of the DC/DC converter (10) after a change rate of a magnitude of a current flowing through a winding of a primary or secondary side of a transformer (TR) with respect to time reaches a value within a predetermined change rate range.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Japanese application serial no. 2016-093908, filed on May 9, 2016, and Japanese application serial no. 2017-045657, filed on Mar. 10, 2017. The entirety of each of the above-mentioned patent applications is hereby incorporated by reference herein and made a part of this specification.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a power conversion device, and more particularly to a power conversion device having a function of converting a voltage in a first range applied to a first input/output terminal pair into a voltage in a second range and outputting the voltage from a second input/output terminal pair and a function of converting a voltage in the second range applied to the second input/output terminal pair into a voltage in the first range and outputting the voltage from the first input/output terminal pair.

Description of Related Art

As a bidirectional DC/DC converter in which large-capacity bidirectional DC conversion is possible, an insulation type bidirectional DC/DC converter (see, for example, Published Japanese Translation No. 2015-517788 of the PCT International Publication) having a configuration in which two full bridge circuits are connected via a transformer is known.

PRIOR ART DOCUMENT Patent Document

[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2015-517788

SUMMARY OF THE INVENTION

If the insulation type bidirectional DC/DC converter (hereinafter referred to as insulation type full bridge converter) having the above configuration performs a step-up operation or a step-down operation, ON/OFF of four switching elements in a full bridge circuit of a primary side is controlled so that a state in which a direction of a current flowing through a winding of the primary side of the transformer is a first direction and a state in which the direction of the current is a second direction opposite to the first direction alternate. Transition timings between the above-described two states are determined irrespective of an amount (magnitude) of the current flowing through the primary side/secondary side. Thus, if a power transmission direction from the insulation type full bridge converter is switched (if input/output terminals previously functioning as input terminals function as output terminals), it is necessary to wait for a certain period of time for protection of an element in the converter or the like.

The present invention has been made in view of the above-described circumstances, and an objective of the present invention is to provide a power conversion device which includes an insulation type full bridge converter and can switch a power transmission direction at a high speed (in a short time).

In order to achieve the above-described objective, a power conversion device of the present invention includes a first input/output terminal pair; a second input/output terminal pair; a DC/DC converter connected to the first input/output terminal pair and the second input/output terminal pair; and a control unit configured to control the DC/DC converter. The DC/DC converter of the power conversion device of the present invention includes a first switching leg having first and second switching elements connected in series via a first connection point and connected to the first input/output terminal pair; a second switching leg having third and fourth switching elements connected in series via a second connection point and connected in parallel to the first switching leg; a third switching leg having fifth and seventh switching elements connected in series via a third connection point and connected to the second input/output terminal pair; a fourth switching leg having sixth and eighth switching elements connected in series via a fourth connection point and connected in parallel to the third switching leg; a first energy storage and conversion unit connected to the first connection point and the second connection point and connected to one winding of a transformer and a first reactor connected in series; and a second energy storage and conversion unit connected to the third connection point and the fourth connection point and connected to the other winding of the transformer and a second reactor connected in series.

Also, the control unit is able to execute a first control for causing the DC/DC converter to convert a voltage within a first range applied to the first input/output terminal pair into a voltage within a second range and to output the voltage within the second range from the second input/output terminal pair and a second control for causing the DC/DC converter to convert a voltage within the second range applied to the second input/output terminal pair into a voltage within the first range and to output the voltage within the first range from the first input/output terminal pair. The first control capable of being executed by the control unit is a control for controlling ON/OFF of each switching element in the DC/DC converter so that a state of the DC/DC converter iteratively transitions between, in an order of, a first state in which a current input from the first input/output terminal pair flows through the first reactor, a second state in which a current is able to circulate along a path including the first reactor, a third state in which the current input from the first input/output terminal pair flows through the first reactor in a direction opposite to a direction in the first state, and a fourth state in which a current is able to circulate along a path including the first reactor while flowing through the first reactor in a direction opposite to a direction in the second state, and is a control for causing the state of the DC/DC converter to be transitioned from the second state to the third state after a first load current value, which is a value of a load current flowing through the first reactor, is within a first current value range if the first load current value is not within the first current value range when the state of the DC/DC converter is to be transitioned from the second state to the third state and causing the state of the DC/DC converter to transition from the fourth state to the first state after the first load current value is within the first current value range if the first load current value is not within the first current value range when the state of the DC/DC converter is to be transitioned from the fourth state to the first state. Also, the second control is a control for controlling ON/OFF of each switching element in the DC/DC converter so that a state of the DC/DC converter iteratively transitions between, in an order of, a fifth state in which a current input from the second input/output terminal pair flows through the second reactor and no current is output from the first input/output terminal pair, a sixth state in which the current input from the second input/output terminal pair flows through the second reactor and a current is output from the first input/output terminal pair, a seventh state in which no current is input from the second input/output terminal pair and no current is output from the first input/output terminal pair, an eighth state in which the current input from the second input/output terminal pair flows through the second reactor in a direction opposite to a direction in the fifth state and no current is output from the first input/output terminal pair, a ninth state in which the current input from the second input/output terminal pair flows through the second reactor in a direction opposite to a direction in the sixth state and a current is output from the first input/output terminal pair, and a tenth state in which no current is input from the second input/output terminal pair and no current is output from the first input/output terminal pair and is a control for causing the state of the DC/DC converter to transition from the sixth state to the seventh state after a second load current value, which is a value of a load current flowing through the second reactor, is within a second current value range if the second load current value is not within the second current value range when the state of the DC/DC converter is to be transitioned from the sixth state to the seventh state and causing the state of the DC/DC converter to transition from the ninth state to the tenth state after the second load current value is within the second current value range if the second load current value is not within the second current value range when the state of the DC/DC converter is to be transitioned from the ninth state to the tenth state. The power conversion device of the present invention has a configuration in which at least one of a determination of whether or not the first load current value in the first control is within the first current value range and a determination of whether or not the second load current value in the second control is within the second current value range is performed on the basis of a pattern of change in a value of a current flowing through the first reactor or the second reactor with respect to time.

That is, for the DC/DC converter, it is effective to reduce a load current value at the time of transition from the second state to the third state, a load current value at the time of transition from the fourth state to the first state, a load current value at the time of transition from the sixth state to the seventh state, and a load current value at the time of transition from the ninth state to the tenth state so as to improve a speed of switching a power transmission direction of the power conversion device in which the first control for causing the state of the DC/DC converter to iteratively transition between the first to fourth states and the second control for causing the state of the DC/DC converter to iteratively transition between the fifth to tenth states are performed. According to the power conversion device of the present invention having the above-described configuration, it is possible to reduce the load current value at each transition time (reduce it to substantially “0”). Therefore, according to the power conversion device of the present invention, the power transmission direction can be switched at a high speed (in a short time) as compared with a power conversion device in which the transition timing to various states is fixed.

The power conversion device of the present invention further includes a current sensor for measuring the value of the current flowing through the first reactor, wherein a configuration in which the control unit determines whether or not the first load current value is within the first current value range on the basis of the change pattern of the value of the current flowing through the first reactor measured by the current sensor with respect to time during the first control, and the control unit determines whether or not the second load current value is within the second current value range by using the value of the current flowing through the first reactor measured by the current sensor as the second load current value during the second control may be adopted.

Also, the power conversion device of the present invention further includes a current sensor for measuring the value of the current flowing through the second reactor, wherein a configuration in which the control unit determines whether or not the second load current value is within the second current value range on the basis of the change pattern of the value of the current flowing through the second reactor measured by the current sensor with respect to time during the second control, and the control unit determines whether or not the first load current value is within the first current value range by using the value of the current flowing through the second reactor measured by the current sensor as the first load current value during the first control may be adopted.

According to the present invention, it is possible to provide a power conversion device which includes an insulation type full bridge converter and can switch a power transmission direction at a high speed (in a short time).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic configuration diagram of a power conversion device according to an embodiment of the present invention.

FIG. 1B is an explanatory diagram of a determination circuit provided in a control unit of the power conversion device according to the embodiment.

FIG. 2 is a timing chart of output control signals G1 to G4 when the control unit provided in the power conversion device according to the embodiment causes a DC/DC converter to operate as a step-down converter whose first input/output terminal side is a primary side.

FIG. 3 is a timing chart of output control signals G1 to G6 when the control unit causes the DC/DC converter to operate as a step-up converter whose first input/output terminal side is a primary side.

FIG. 4 is a time chart illustrating changes in currents flowing through parts of the DC/DC converter with respect to time when the control unit causes the DC/DC converter 10 to function as a step-down converter whose first input/output terminal side is the primary side together with changes in the control signals G1 to G4 with respect to time.

FIG. 5A is an explanatory diagram illustrating current paths of a primary side and a secondary side of the DC/DC converter when the control unit causes the DC/DC converter to function as a step-down converter whose first input/output terminal side is the primary side.

FIG. 5B is an explanatory diagram of the current paths of the primary side and the secondary side of the DC/DC converter following FIG. 5A.

FIG. 5C is an explanatory diagram of the current paths of the primary side and the secondary side of the DC/DC converter following FIG. 5B.

FIG. 5D is an explanatory diagram of the current paths of the primary side and the secondary side of the DC/DC converter following FIG. 5C.

FIG. 5E is an explanatory diagram of the current paths of the primary side and the secondary side of the DC/DC converter following FIG. 5D.

FIG. 5F is an explanatory diagram of the current paths of the primary side and the secondary side of the DC/DC converter following FIG. 5E.

FIG. 6 is a flowchart of a step-down control process executed by the control unit.

FIG. 7 is a diagram illustrating the significance of the processing of steps S104 and S109 of the step-up control process.

FIG. 8 is a flowchart of a step-up control processing executed by the control unit.

FIG. 9 is a time chart illustrating changes in currents flowing through parts of the DC/DC converter with respect to time when the control unit causes the DC/DC converter to function as a step-up converter whose first input/output terminal side is the primary side together with changes in control signals G1 to G6 with respect to time.

FIG. 10A is an explanatory diagram of current paths of a primary side and a secondary side of a DC/DC converter when the control unit causes the DC/DC converter to function as a step-up converter whose first input/output terminal side is the primary side.

FIG. 10B is an explanatory diagram of current paths of the primary side and the secondary side of the DC/DC converter following FIG. 10A.

FIG. 10C is an explanatory diagram of the current paths of the primary side and the secondary side of the DC/DC converter following FIG. 10B.

FIG. 10D is an explanatory diagram of the current paths of the primary side and the secondary side of the DC/DC converter following FIG. 10C.

FIG. 10E is an explanatory diagram of the current paths of the primary side and the secondary side of the DC/DC converter following FIG. 10D.

FIG. 10F is an explanatory diagram of the current paths of the primary side and the secondary side of the DC/DC converter following FIG. 10E.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1A is a schematic configuration diagram of a power conversion device according to an embodiment of the present invention.

The power conversion device according to the present embodiment is a device in which bidirectional power conversion is possible. As illustrated, the power conversion device includes a DC/DC converter 10, a control unit 20, and two pairs of input/output terminals 13 (13 p and 13 m). In any pair of input/output terminals 13, the input/output terminal 13 p is an input/output terminal of a high potential side and the input/output terminal 13 m is an input/output terminal of a low potential side.

The DC/DC converter 10 is an insulation type bidirectional DC/DC converter including a transformer TR, two reactors Lr, and two full bridge circuits 11 as main components. Hereinafter, the full bridge circuit 11 on the left side and the full bridge circuit 11 on the right side in FIG. 1A are referred to as a first full bridge circuit 11 and a second full bridge circuit 11, respectively. Likewise, the reactors Lr on the left side and the right side in FIG. 1A are referred to as a first reactor and a second reactor, respectively, and a winding on the left side and a winding on the right side in the transformer TR in FIG. 1A are referred to as a first winding and a second winding, respectively. Also, the input/output terminals 13 (13 p and 13 m) on the left side and the right side in FIG. 1A are referred to as a first input/output terminal 13 and a second input/output terminal 13, respectively. Current sensors 15 on the left side and the right side in FIG. 1A are referred to as a first current sensor 15 and a second current sensor 15, respectively. The transformer TR of the DC/DC converter 10 may not have a turn ratio of 1:1. However, the configuration and operation of the power conversion device will be described below under the assumption that the turn ratio of the transformer TR is 1:1.

The first full bridge circuit 11 of the DC/DC converter 10 includes a first switching leg having a first switching element SW1 and a second switching element SW2 connected in series and a second switching leg having a third switching element SW3 and a fourth switching element SW4 connected in series. As illustrated, a diode Dn is connected in parallel between terminals of an n^(th) switching element SWn (n=1 to 4) of each switching leg. Also, each switching leg is connected to a pair of first input/output terminals 13 and a connection point between the first and second switching elements SW1 and SW2 of the first switching leg is connected to one end of the first winding of the transformer TR via the first reactor. A connection point between the third and fourth switching elements SW3 and SW4 of the second switching leg is connected to the other end of the first winding of the transformer TR.

The second full bridge circuit 11 of the DC/DC converter 10 includes a third switching leg having a fifth switching element SW5 and a seventh switching element SW7 connected in series and a fourth switching leg having a sixth switching element SW6 and an eighth switching element SW8 connected in series. As illustrated, a diode Dn is connected in parallel between terminals of an n^(th) switching element SWn (n=5 to 8) of each switching leg. Both the third switching leg and the fourth switching leg are connected to the second input/output terminal pair. A connection point between the fifth and seventh switching elements SW5 and SW7 of the third switching leg is connected to one end of the second winding of the transformer TR via the second reactor, and a connection point between the sixth and eighth switching elements SW6 and SW8 of the fourth switching leg is connected to the other end of the second winding of the transformer TR.

Two current sensors 15 for measuring the magnitude of the current flowing through each reactor Lr are attached to the DC/DC converter 10. Various sensors (not illustrated) for measuring magnitudes of an input/output voltage and an input/output current are also attached to the DC/DC converter 10.

The control unit 20 is a unit that controls the DC/DC converter 10 (ON/OFF of each switching element in the DC/DC converter 10) by changing the level of the control signal for each switching element in the DC/DC converter 10. Hereinafter, a control signal for the n^(th) switching element SWn (n=1 to 8) is referred to as a control signal Gn.

The control unit 20 is a unit for controlling the DC/DC converter 10. The control unit 20 includes a processor (a microcontroller or the like), a gate driver, two determination circuits 22, and the like, and an output of a first current sensor 15 is input to one determination circuit 22 (hereinafter referred to as a first determination circuit 22). Also, an output of a second current sensor 15 is input to the other determination circuit 22 (hereinafter referred to as a second determination circuit 22).

In FIG. 1B, a configuration of the determination circuit 22 is illustrated. As illustrated, the determination circuit 22 includes a low pass filter (LPF) 25 for removing a low frequency component from an output of the current sensor 15, a differential circuit 26 that differentiates an output of the LPF 25, and a window comparator 27 that outputs a determination result of whether or not an output of the differential circuit 26 is a value within a range from −Vth to Vth. A field of application of each determination circuit 22 and reference voltages −Vth and Vth input to the window comparator 27 of each determination circuit 22 will be described below.

Returning to FIG. 1A, description of the control unit 20 will be continued.

The control unit 20 determines whether to operate the DC/DC converter 10 as one of converters of the following four types on the basis of input data (a current value and a voltage value) and is configured (programmed) to control the DC/DC converter 10 so that the DC/DC converter operates as the determined converter.

-   -   A step-up converter whose first input/output terminal 13 side is         the primary side.     -   A step-down converter whose first input/output terminal 13 side         is the primary side.     -   A step-up converter whose second input/output terminal 13 side         is the primary side.     -   A step-down converter whose second input/output terminal 13 side         is the primary side.

Also, the control unit 20 is configured (programmed) so that a change in control details for the DC/DC converter 10 (a change from the control for causing the DC/DC converter 10 to operate as the step-up converter whose first input/output terminal 13 side is the primary side to the control for causing the DC/DC converter 10 to operate as the step-down converter whose second input/output terminal 13 side is the primary side or the like) is immediately performed.

Hereinafter, the configuration and operation of the power conversion device according to the present embodiment will be specifically described.

First, details of basic control of the DC/DC converter 10 by the control unit 20 will be described. Details of the control performed on the first and second full bridge circuits 11 by the control unit 20 when the DC/DC converter 10 is operated as the step-up/step-down converter whose second input/output terminal 13 side is the primary side are the same as details of the control performed on the second and first full bridge circuits 11 by the control unit 20 when the DC/DC converter 10 is operated as the step-up/step-down converter whose first input/output terminal 13 side is the primary side. In other words, the control details of the control unit 20 when the DC/DC converter 10 is operated as the step-up/step-down converter whose second input/output terminal 13 side is the primary side are substantially the same as the control details of the control unit 20 when the DC/DC converter 10 is operated as the step-up/step-down converter whose first input/output terminal 13 side is the primary side. Thus, only the control details of the control unit 20 when the DC/DC converter 10 is operated as the step-up/step-down converter whose first input/output terminal 13 side is the primary side will be described.

Also, control signals G1 to G8 actually output by the control unit 20 are configured so that ON and OFF of the two switching elements (ON of the first switching element SW1, OFF of the second switching element SW2, and the like) are performed with a time difference (so-called dead time). However, the operation of the control unit 20 will be described below under the assumption that the time difference is not assigned to avoid complicated explanation.

(1) When the DC/DC converter 10 functions as a step-down converter whose first input/output terminal 13 side is the primary side:

In this case, the control unit 20 basically outputs control signals G1 to G4 which change with time as illustrated in FIG. 2.

In other words, when the DC/DC converter 10 functions as the step-down converter whose first input/output terminal 13 side is the primary side, the control unit 20 outputs the control signal G1 whose ON time is 1/2 of a period T, i.e., the control signal G1 having a duty ratio of 1/2. Also, the control unit 20 outputs the control signal G2 obtained by inverting the control signal G1, i.e., the control signal G2 by which the second switching element SW2 is turned OFF/ON when the first switching element SW1 is ON/OFF. Further, the control unit 20 outputs the control signal G3 whose phase is shifted by θ from the control signal G1 and the control signal G4 obtained by inverting the control signal G3.

Then, while the control signals G1 to G4 as described above are output, the control unit 20 changes the value of 0 so that the output voltage or the output current of the DC/DC converter 10 become a target value.

Also, when the power conversion device according to the present embodiment operates as the step-down device whose first input/output terminal 13 side is the primary side, the second full bridge circuit 11 (the full bridge circuit 11 of the secondary side; see FIG. 1A) is used as a full wave rectifier (diode bridge circuit). Accordingly, when the DC/DC converter 10 is operated as the step-down converter whose first input/output terminal 13 side is the primary side, the control unit 20 maintains the state of each switching element in the second full bridge circuit 11 in an OFF state.

(2) When the DC/DC converter 10 functions as a step-up converter whose first input/output terminal 13 side is the primary side: In this case, the control unit 20 basically outputs the control signals G1 to G6 which change with time as illustrated in FIG. 3.

In other words, when the DC/DC converter 10 functions as the step-up converter whose first input/output terminal 13 side is the primary side, the control unit 20 outputs the control signal G2 having a duty ratio of 1/2 and the control signal G1 which rises at a falling edge time of the control signal G2 and has a shorter ON time than the control signal G2. Also, the control unit 20 outputs the control signals G3 and G4, which are signals obtained by delaying the control signals G1 and G2 by T/2, respectively. Further, the control unit 20 outputs the control signal G5 having a high level only during a time Ton from the falling edge time of the control signal G1 and the control signal G6 having a high level only during the time Ton from the falling edge time of the control signal G3.

Then, while the control signals G1 to G6 as described above are output, the control unit changes a value of the time Ton so that the output voltage or the output current of the DC/DC converter 10 becomes the target value.

Hereinafter, details of control of the DC/DC converter 10 by the control unit 20 will be more specifically described.

First, an operation of the control unit 20 when the DC/DC converter 10 functions as a step-down converter whose first input/output terminal 13 side is the primary side will be described.

FIG. 4 illustrates changes in currents flowing through parts of the DC/DC converter 10 with respect to time when the control unit 20 causes the DC/DC converter 10 to function as the step-down converter whose first input/output terminal 13 side is the primary side together with changes in the control signals G1 to G4 with respect to time. Also, FIGS. 5A to 5F are explanatory diagrams illustrating current paths of a primary side and a secondary side of the DC/DC converter 10 when the control unit 20 causes the DC/DC converter 10 to function as the step-down converter whose first input/output terminal 13 side is the primary side. Also, in FIG. 4 and FIG. 9 described below, an “input current” and an “output current” are a current flowing into the first input/output terminal 13 p and a current flowing out from the second input/output terminal 13 p, respectively. Also, an “input side Lr current” and an “output side Lr current” are a current flowing through the first reactor (the reactor Lr of the primary side) and a current flowing through the second reactor (the reactor Lr of the secondary side).

As described above, when the DC/DC converter 10 is operated as the step-down converter whose first input/output terminal 13 side is the primary side, the control unit 20 outputs the control signals G1 to G4 that change with time as illustrated in FIG. 4.

Accordingly, when the control unit 20 operates the DC/DC converter 10 as the step-down converter whose first input/output terminal 13 side is the primary side, the state of the DC/DC converter 10 iteratively transitions in the order of state #1, state #2, state #3, and state #4 between the following four states.

-   -   State #1: a state in which the first switching element SW1 and         the fourth switching element SW4 are ON (the first switching         element SW1 and the fourth switching element SW4 are ON and the         other switching elements are OFF; this is the same in the         following).     -   State #2: a state in which the second switching element SW2 and         the fourth switching element SW4 are ON.     -   State #3: a state in which the second switching element SW2 and         the third switching element SW3 are ON.     -   State #4: a state in which the first switching element SW1 and         the third switching element SW3 are ON.

When the state of the DC/DC converter 10 is in state #1, as illustrated in FIG. 5A, a current flows through a path of first input/output terminal 13 p→first switching element SW1→first reactor→transformer TR→fourth switching element SW4→first input/output terminal 13 m. Accordingly, energy is stored in the first reactor and the input side Lr current rises as illustrated in FIG. 4. Also, energy is transmitted to the secondary side via the transformer TR and rectified by the second full bridge circuit 11 and output from the second input/output terminal 13 as illustrated in FIGS. 4 and 5A.

When the state of the DC/DC converter 10 transitions to state #2, the first switching element SW1 is OFF and the second switching element SW2 is ON. Accordingly, as illustrated in FIG. 5B, a current circulates along a path of second switching element SW2 (and diode D2)→first reactor→transformer TR→fourth switching element SW4 according to energy stored in the first reactor. As a result, the energy stored in the first reactor is transmitted to the secondary side, rectified by the second full bridge circuit 11, and output from the second input/output terminal 13.

The magnitude of the circulating current in state #2 decreases with the movement of the energy stored in the first reactor to the secondary side. Therefore, when a certain time (time corresponding to the θ value) has elapsed after the transition to state #2, as illustrated in FIG. 5C, a state where no current flows through the second reactor is formed. Also, when a magnitude of a load current of the secondary side of the transformer TR (i.e., a current flowing through the second reactor) is “0”, a magnitude of a load current of the primary side of the transformer TR is also “0.” Because the current flowing through the primary side of the transformer TR is the load current+the excitation current, a current circulating through the primary side of the DC/DC converter 10 is the excitation current in FIG. 5C.

The DC/DC converter 10 is configured so that the state is a state illustrated in FIG. 5C in principle (if a load is not excessively large) when the original timing of transition to state #3 has been reached. Here, the original transition timing is a transition timing when a timing adjustment function (of which details will be described below) provided in the control unit 20 is not working.

When the state of the DC/DC converter 10 is state #3 (i.e., when SW2 and SW3 are ON), as illustrated in FIG. 5D, a current flows through a path of first input/output terminal 13 p→third switching element SW3→transformer TR→first reactor→second switching element SW2→first input/output terminal 13 m. In other words, a current flows through the first reactor in a direction opposite to that in a case in which the state of the DC/DC converter 10 is state #1 (FIG. 5A).

Accordingly, energy is stored in the first reactor (see FIG. 4) and energy is transmitted to the secondary side via the transformer TR, rectified by the second full bridge circuit 11, and output from the second input/output terminal 13.

When the primary side circuit of the DC/DC converter 10 transitions from state #3 to state #4, as illustrated in FIG. 5E, a current circulates along a path of third switching element SW3→transformer TR→first reactor→first switching element SW1 (and diode D1) according to the energy stored in the first reactor. Thus, the energy stored in the first reactor is transmitted to the secondary side.

The magnitude of the circulating current in this state #4 also decreases with movement of energy stored in the first reactor to the secondary side. When a certain time has elapsed after the transition to state #4, a state in which no current flows through the second reactor, i.e., a state in which load currents of the primary side and the secondary side do not flow, is formed as illustrated in FIG. 5F. Even when the original timing of transition from state #4 to state #1 has been reached, the DC/DC converter 10 is configured to make the state be a state illustrated in FIG. 5F in principle.

As described above, when the original timing of transition from the state (state #2 or state #4) in which the current circulates in the circuit on the primary side to a next state has been reached, the DC/DC converter 10 is configured so that the magnitude of the second reactor current becomes “0.” However, it is difficult to configure the DC/DC converter 10 so that the above-described condition is satisfied without degrading the performance of the power conversion device in all operation conditions of the power conversion device.

Thus, when the control unit 20 (the microcontroller in the control unit 20) causes the DC/DC converter 10 to operate as the step-down converter whose first input/output terminal 13 side is the primary side, the control unit 20 is configured (programmed) so as to perform the step-down control process of a procedure illustrated in FIG. 6.

That is, the control unit 20 that started the step-down control process first performs a timer setting process of setting a specified value in an interrupt timer in step S100. Here, the interrupt timer is a timer that generates an interruption when a time according to a set value has elapsed. Also, a specified value is a value at which the interrupt timer will generate an interruption after the passage of a time T/2 (T is a period of the control signal: see FIG. 2) if the value is set.

Further, in step S100, the control unit 20 also performs a process of turning the first switching element SW1 and the fourth switching element SW4 OFF by changing the levels of the control signals G1 and G4 to a high level.

Upon completion of the processing of step S100, the control unit 20 waits (monitors) an SW1 and SW2 control timing in step S101. Here, the SW1 and SW2 control timing is a timing determined by a θ value to control (change) the ON/OFF states of the first switching element SW1 and the second switching element SW2. The processing of step S101 to be first executed after the start of the step-down control process may be performed irrespective of the θ value (e.g., a process of determining that the SW1 and SW2 control timing has been reached when a predetermined time has elapsed from a time point at which step S100 was executed). Also, the processing of step S101 of the second and subsequent times may be a process of determining whether or not the SW1 and SW2 control timing has been reached on the basis of an elapsed time after the occurrence of an interruption or a process of determining whether or not the SW1 and SW2 control timing has been reached on the basis of an elapsed time after the first switching element SW1 was previously turned ON.

When the SW1 and SW2 control timing has been reached (step S101; YES), the control unit 20 changes the levels of the control signals G1 and G2 to turn the first switching element SW1 OFF and turn the second switching element SW2 ON (step S102). In this step S102, the control unit 20 assigns a time difference to ON/OFF of the first switching element SW1 and the second switching element SW2 to suppress a through-current. More specifically, the control unit 20 changes the level of the control signal G2 when a predetermined time has elapsed after a change in the level of the control signal G1.

Thereafter, the control unit 20 waits (monitors) the occurrence of an interruption in step S103. When the interruption has occurred (step S103; YES), the second unit 20 determines whether or not a first change rate, which is a change rate in a value of a current flowing through the first reactor with respect to time, is within a first change rate range (step S104). Here, the first change rate range is a determined change rate range centered on “0” so that a power transmission direction can be switched at a desired speed if the processing of steps S105 and S110 (of which details will be described below) is performed when the first change rate is within the range.

During the step-down control process, the window comparator 27 of the first determination circuit 22 receives an output voltage of the differential circuit 26 when the first change rate is a lower limit value of the first change rate range and an output voltage of the differential circuit 26 when the first change rate is an upper limit value of the first change rate range as the reference voltages −Vth and Vth (see FIG. 1B). That is, the first determination circuit 22 functions as a circuit that outputs a signal indicating whether or not the first change rate is within the first change rate range during the step-down control process. In step S104, the control unit 20 determines whether or not the first change rate is within the first change rate range on the basis of a signal from the first determination circuit 22.

If the first change rate is within the first change rate range (step S104; YES), the control unit 20 sets a specified value in the interrupt timer (step S105). Also, the control unit 20 turns the fourth switching element SW4 OFF and turns the third switching element SW3 ON by changing the levels of the control signals G3 and G4 (step S105). Also, if the first change rate is not within the first change rate range (step S104; NO), the control unit 20 waits for the first change rate to be a value within the first change rate range by iterating the processing of step S104, and then performs the processing of S105.

Also, in step S105 and steps S107 and S110 to be described later, similar to the execution of the above-described step S102, the control unit 20 assigns a time difference to ON/OFF of the two switching elements to suppress a through-current.

Upon completion of the processing of step S105, the control unit 20 waits (monitors) the SW1 and SW2 control timing (step S106). The SW1 and SW2 control timing in this step S106 is also a timing determined by the θ value and a Ton value to control (change) the ON/OFF states of the first switching element SW1 and the second switching element SW2. The processing of step S106 may be a process of determining whether or not the SW1 and SW2 control timing has been reached on the basis of the passage of time after the first switching element SW1 was previously turned OFF or a process of determining whether or not the SW1 and SW2 control timing has been reached on the basis of the passage of time after the occurrence of an interruption.

When the SW1 and SW2 control timing has been reached (step S106; YES), the control unit 20 changes the levels of the control signals G1 and G2 to turn the first switching element SW1 ON and turn the second switching element SW2 OFF (step S107).

Thereafter, the control unit 20 waits for occurrence of an interruption in step S108. If the interruption has occurred (step S108; YES), the control unit 20 determines whether or not the first change rate at an interruption time is within the first change rate range on the basis of a signal from the first determination circuit 22 (step S109).

If the first change rate is within the first change rate range (step S109; YES), the control unit 20 sets a specified value in the interrupt timer, turns the third switching element SW3 OFF, and turns the fourth switching element SW4 ON (step S110). Also, if the first change rate is not within the first change rate range (step S109; NO), the control unit 20 waits for the first change rate to be within the first change rate range, and then performs the processing of step S110.

Then, the control unit 20 that has completed the processing of step S110 resumes step S101 and the processing subsequent thereto.

Hereinafter, the significance of the processing of steps S104 and S109 in the step-down control process (FIG. 6) will be described.

To switch the power transmission direction at a high speed by the DC/DC converter 10, it is only necessary to perform the processing of steps S105 and S110 after a load current value of the primary side of the transformer TR becomes small during the step-down control process. However, a large excitation current according to an input voltage flows through the primary side of the transformer TR. Because a current value measured by the first current sensor 15 is (excitation current value+load current value), a state in which the load current value decreases to a desired value may not be detected by merely comparing a current value measured by the first current sensor 15 with a threshold value.

In steps S104 and S109, it is determined whether or not the first change rate is within the first change rate range to prevent such a malfunction from occurring. Specifically, a current value measured by the first current sensor 15 changes with time as illustrated in FIG. 7, but a change rate of an excitation current value is significantly low. Thus, if a load current component is not included in a result of measuring a current value from the first current sensor 15, a change rate of the current value with respect to time is substantially “0” as illustrated in FIG. 7. Accordingly, it is possible to determine whether or not the load current value of the primary side of the transformer TR is significantly small (substantially “0”) according to whether or not the first change rate is within the first change rate range.

Next, an operation of the control unit 20 when the DC/DC converter 10 functions as the step-up converter whose first input/output terminal 13 side is the primary side will be described.

If the DC/DC converter 10 is operated as the step-up converter whose first input/output terminal 13 side is the primary side, the control unit 20 is configured (programmed) to perform the step-up control process of the procedure illustrated in FIG. 8.

In other words, the control unit 20 that started the step-up control process firstly sets a specified value in the interrupt timer (step S200). Also, in step S200, the control unit 20 performs a process of turning the first switching element SW1, the fourth switching element SW4, and the fifth switching element SW5 ON by changing the levels of the control signals G1, G4, and G5 to a high level.

The control unit 20 that has completed the processing of step S200 waits (monitors) an SW5 control timing in step S201. Here, the SW5 control timing is a timing determined by the Ton value and at which the fifth switching element SW5 is to be turned OFF such that it is determined by the Ton value.

When the SW5 control timing has been reached (step S201; YES), the control unit 20 turns the fifth switching element SW5 OFF by changing the level of the control signal G5 (step S202).

Thereafter, the control unit 20 determines whether or not the first change rate is within a second change rate range (step S203). Here, the second change rate range is a determined change rate range centered on “0” so that the power transmission direction can be switched at a desired speed if the processing of steps S204 and S212 is performed when the first change rate is within the range.

During the step-up control process, the window comparator 25 of the first determination circuit 22 receives an output voltage of the differential circuit 26 when the first change rate is the lower limit value of the second change rate range and an output voltage of the differential circuit 26 when the first change rate is the upper limit value of the second change rate range as the reference voltage −Vth and Vth (see FIG. 1B). In other words, the first determination circuit 22 functions as a circuit that outputs a signal indicating whether or not the first change rate is within the second change rate range during the step-up control process. In step S203, the control unit 20 determines whether or not the first change rate is within the second change rate range on the basis of a signal from the first determination circuit 22.

If the first change rate is not within the second change rate range (step S203; NO), the control unit 20 iterates the processing (determination) of step S203. If the first change rate is within the second change rate range (step S203; YES), the control unit 20 turns the first switching element SW1 OFF (step S204). Next, the control unit 20 waits for occurrence of an interruption (step S205). If the interruption has occurred (step S205; YES), the control unit 20 sets a specified value in the interruption timer, turns the fourth switching element SW4 OFF, and turns the second switching element SW2, the third switching element SW3, and the sixth switching element SW6 ON (step S206). Also, if the interruption has already occurred during the execution of step S203 (or step S204), the control unit 20 performs the processing of step S206 without waiting for the occurrence of the interruption in step S205.

The control unit 20 that has completed the processing of step S206 awaits (monitors) an SW6 control timing in step S207. Here, the SW6 control timing is a timing determined by the Ton value and at which the sixth switching element SW6 is to be turned OFF such that it is determined by the Ton value.

When the SW6 control timing has been reached (step S207; YES), the control unit 20 turns the sixth switching element SW6 OFF by changing the level of the control signal G6 (step S208).

Thereafter, the control unit 20 determines whether or not the first change rate is within the second change rate range (step S209). The processing of step S209 is the same as the processing of step S204.

If the first change rate is not within the second change rate range (step S209; NO), the control unit 20 iterates the processing (determination) of step S209. If the first change rate is within the second change rate range (step S209; YES), the control unit 20 turns the third switching element SW3 OFF (step S210). Next, the control unit 20 waits for occurrence of an interruption (step S211). If the interruption has occurred (step S211; YES), the control unit 20 sets a specified value in the interruption timer, turns the second switching element SW2 OFF, and turns the first switching element SW1, the fourth switching element SW4, and the fifth switching element SW5 ON (step S212). If the interruption has already occurred during the execution of step S209 (or step S210), the control unit 20 performs the processing of step S212 without waiting for the occurrence of the interruption in step S211.

Then, the control unit 20 that has completed the processing of step S212 resumes a process of moving to step S201.

Hereinafter, details of the step-up control process will be more specifically described.

FIG. 9 illustrates changes in currents flowing through parts of the DC/DC converter 10 with respect to time when the control unit 20 causes the DC/DC converter 10 to function as a step-up converter whose first input/output terminal 13 side is the primary side together with changes in control signals G1 to G6 with respect to time. Also, FIGS. 10A to 10F are explanatory diagrams of current paths of a primary side and a secondary side of the DC/DC converter 10 when the control unit 20 causes the DC/DC converter 10 to function as the step-up converter whose first input/output terminal 13 side is the primary side.

When the step-up control process of the above-described details is performed, the state of the DC/DC converter 10 iteratively transitions between the following six states in the order of state #5, state #6, state #7, state #8, state #9, and state #10 (see FIG. 9).

-   -   State #5: a state in which the first switching element SW1, the         fourth switching element SW4, and the fifth switching element         SW5 are ON.     -   State #6: a state in which the first switching element SW1 and         the fourth switching element SW4 are ON.     -   State #7: a state in which the fourth switching element SW4 is         ON.     -   State #8: a state in which the second switching element SW2, the         third switching element SW3, and the sixth switching element SW6         are ON.     -   State #9: a state in which the second switching element SW2 and         the third switching element SW3 are ON.     -   State #10: a state in which the second switching element SW2 is         ON.

When the state of the DC/DC converter 10 is state #5 (i.e., when SW1, SW4, and SW5 are ON), as illustrated in FIG. 10A, a current flows through a path of first input/output terminal 13 p→first switching element SW1→first reactor→transformer TR→fourth switching element SW4→first input/output terminal 13 m. Accordingly, energy is stored in the first reactor and the input side Lr current rises as illustrated in FIG. 9. Also, energy is transmitted to the secondary side via the transformer TR. However, because the fifth switching element SW5 of the second full bridge circuit 11 is ON, as illustrated in FIG. 10A, a current circulates along a path of diode D6→transformer TR→second reactor→fifth switching element SW5 on the secondary side of the DC/DC converter 10. Accordingly, energy from the primary side is not output from the second input/output terminal 13, but stored in the second reactor.

When the fifth switching element SW5 is OFF and the state of the DC/DC converter 10 becomes state #6, as illustrated in FIG. 10B, the energy stored in the first reactor and energy from a power supply are transmitted to the secondary side and the current rectified by the second full bridge circuit 11 is output from the second input/output terminal 13.

Because the output voltage of the load is applied to the second input/output terminal 13, the input current, the input side Lr current, and the output current gradually decrease if the state of the DC/DC converter 10 is state #6. Then, when it is detected that the first change rate is within the first change rate range in step S203 (FIG. 7), the first switching element SW1 is OFF and the state of the DC/DC converter 10 transitions to state #7. As a result, as illustrated in FIG. 10C, a state in which no load current flows through the primary side and the secondary side of the DC/DC converter 10 is formed. As the state illustrated in FIG. 10C, a state in which no load current flows through the primary side and the secondary side of the DC/DC converter 10 is formed.

When the state of the DC/DC converter 10 transitions from state #7 to state #8, as illustrated in FIG. 10D, a current flows through a path of first input/output terminal 13 p→third switching element SW3→transformer TR→first reactor→second switching element SW2→first input/output terminal 13 m. Accordingly, energy is stored in the first reactor (see FIG. 9), and energy is transmitted to the secondary side via the transformer TR. However, because the sixth switching element SW6 is ON, as illustrated in FIG. 10D, a current circulates along a path of diode D5→second reactor→transformer TR→sixth switching element SW6 on the secondary side of the DC/DC converter 10. Accordingly, in state #8, the energy from the primary side is stored in the second reactor.

If the sixth switching element SW6 is OFF and the state of the DC/DC converter 10 is shifted to state #9, energy stored in the second reactor and energy from the power supply are transmitted to the secondary side and a current rectified by the second full bridge circuit 11 is output from the second input/output terminal 13 as illustrated in FIG. 10E.

An output voltage of a load is applied to the second input/output terminal 13. Thus, the input current, the input Lr current, and the output current in state #9 gradually decrease. Then, if the second Lr current value “0” is detected in step S209 (FIG. 8), the third switching element SW3 is turned OFF to be in state #10. Accordingly, when the state of the DC/DC converter 10 is state #10, as illustrated in FIG. 10F, a state in which no load current flows through the primary side and the secondary side of the DC/DC converter 10 is formed.

As described above, in the power conversion device according to the present embodiment, the transition from the second state to the third state and the transition from the fourth state to the first state are performed for the step-down time when the first change rate has reached a value within the first change rate range (the load current value of the primary side has reached substantially “0”). Also, the transition from the sixth state to the seventh state and the transition from the ninth state to the tenth state are performed for the step-up time when the first change rate has reached a value within the second change rate range (the load current value of the primary side has reached “0”). For the DC/DC converter 10, it is effective to reduce a load current value at the time of transition from the second state to the third state, a load current value at the time of transition from the fourth state to the first state, a load current value at the time of transition from the sixth state to the seventh state, and a load current value at the time of transition from the ninth state to the tenth state so as to improve a speed of switching a power transmission direction of the power conversion device in which a control for causing the state of the DC/DC converter 10 to iteratively transition between the first to fourth states and control for causing the state of the DC/DC converter 10 to iteratively transition between the fifth to tenth states are performed. Accordingly, the power conversion device of the present invention can switch the power transmission direction at a higher speed (in a shorter time) than a conventional power conversion device.

Modified Embodiments

The power conversion device according to the above-described embodiment can have various modifications. For example, the power conversion device can be modified to a device which does not include the determination circuit 22 and in which it is determined whether or not the first change rate has reached a value within the first/second change rate range in software. Also, the second determination circuit 22 outputs the same signal as a signal output by the first determination circuit 22 in principle. Accordingly, if the second current sensor 15 and the second determination circuit 22 are removed and the second input/output terminal 13 is the primary side, the timing of transition to various states may be determined on the basis of the output of the first determination circuit 22. Of course, if the first current sensor 15 and the first determination circuit 22 are removed and the first input/output terminal 13 side is the primary side, the timing of transition to various states may be determined on the basis of the output of the second determination circuit 22.

There is a proportional relationship between a load current value of the primary side of the transformer TR and a load current value of the secondary side. A current value detected by the current sensor 15 of the secondary side is the load current value of the secondary side. Accordingly, if the second current sensor 15 and the second determination circuit 22 are removed and the second input/output terminal 13 side is the primary side, the timing of transition to various states may be determined on the basis of the output of the second current sensor 15.

It is only necessary for the power conversion device to have a function of converting a voltage within the first range applied between the first input/output terminals 13 into a voltage within the second range and outputting the voltage from the second input/output terminal 13 and a function of converting a voltage within the second range applied between the second input/output terminals 13 into a voltage within the first range and outputting the voltage from the first input/output terminal 13. Accordingly, according to a combination of the first range and the second range, a unit 20 having only a function of causing the DC/DC converter 10 to operate as a step-up (or step-down) converter whose first input/output terminal 13 side is the primary side or a step-down (or step-up) converter whose second input/output terminal 13 side is the primary side may be adopted as the control unit 20.

Also, the procedure of control of the DC/DC converter 10 by the control unit 20 need not be the same as that described above. For example, although the control unit 20 ascertains a control timing with a so-called software interruption, the control unit 20 that ascertains a control timing by a hardware interruption and re-sets the hardware interruption when it waits for the change rate to be a value within the change rate range may be adopted. 

What is claimed is:
 1. A power conversion device comprising: a first input/output terminal pair; a second input/output terminal pair; a DC/DC converter connected to the first input/output terminal pair and the second input/output terminal pair; and a control unit configured to control the DC/DC converter, wherein the DC/DC converter comprises: a first switching leg having first and second switching elements connected in series via a first connection point and connected to the first input/output terminal pair; a second switching leg having third and fourth switching elements connected in series via a second connection point and connected in parallel to the first switching leg; a third switching leg having fifth and seventh switching elements connected in series via a third connection point and connected to the second input/output terminal pair; a fourth switching leg having sixth and eighth switching elements connected in series via a fourth connection point and connected in parallel to the third switching leg; a first reactor connected to a first winding of a transformer in series, wherein the first connection point is connected to one end of the first winding of the transformer via the first reactor and the second connection point is connected to the other end of the first winding of the transformer; and a second reactor connected to a second winding of the transformer in series, wherein the third connection point is connected to one end of the second winding of the transformer via the second reactor and the fourth connection point is connected to the other end of the second winding of the transformer, wherein the control unit is able to execute a first control for causing the DC/DC converter to convert a voltage within a first range applied to the first input/output terminal pair into a voltage within a second range and to output the voltage within the second range from the second input/output terminal pair and a second control for causing the DC/DC converter to convert a voltage within the second range applied to the second input/output terminal pair into a voltage within the first range and to output the voltage within the first range from the first input/output terminal pair, wherein the first control is a control for controlling ON/OFF of each switching element in the DC/DC converter so that a state of the DC/DC converter iteratively transitions between, in an order of, a first state in which a current input from the first input/output terminal pair flows through the first reactor, a second state in which a current is able to circulate along a path including the first reactor, a third state in which the current input from the first input/output terminal pair flows through the first reactor in a direction opposite to a direction in the first state, and a fourth state in which a current is able to circulate along a path including the first reactor while flowing through the first reactor in a direction opposite to a direction in the second state, and is a control for causing the state of the DC/DC converter to transition from the second state to the third state after a first load current value, which is a value of a load current flowing through the first reactor, is within a first current value range if the first load current value is not within the first current value range when the state of the DC/DC converter is to be transitioned from the second state to the third state and causing the state of the DC/DC converter to transition from the fourth state to the first state after the first load current value is within the first current value range when the state of the DC/DC converter is to be transitioned from the fourth state to the first state, wherein the second control is a control for controlling ON/OFF of each switching element in the DC/DC converter so that a state of the DC/DC converter iteratively transitions between, in an order of, a fifth state in which a current input from the second input/output terminal pair flows through the second reactor and no current is output from the first input/output terminal pair, a sixth state in which the current input from the second input/output terminal pair flows through the second reactor and a current is output from the first input/output terminal pair, a seventh state in which no current is input from the second input/output terminal pair and no current is output from the first input/output terminal pair, an eighth state in which the current input from the second input/output terminal pair flows through the second reactor in a direction opposite to a direction in the fifth state and no current is output from the first input/output terminal pair, a ninth state in which the current input from the second input/output terminal pair flows through the second reactor in a direction opposite to a direction in the sixth state and a current is output from the first input/output terminal pair, and a tenth state in which no current is input from the second input/output terminal pair and no current is output from the first input/output terminal pair in this order and is a control for causing the state of the DC/DC converter to transition from the sixth state to the seventh state after a second load current value, which is a value of a load current flowing through the second reactor, is within a second current value range if the second load current value is not within the second current value range when the state of the DC/DC converter is to be transitioned from the sixth state to the seventh state and causing the state of the DC/DC converter to transition from the ninth state to the tenth state after the second load current value is within the second current value range if the second load current value is not within the second current value range when the state of the DC/DC converter is to be transitioned from the ninth state to the tenth state, and wherein at least one of a determination of whether or not the first load current value in the first control is within the first current value range and a determination of whether or not the second load current value in the second control is within the second current value range is performed on the basis of a pattern of change in a value of a current flowing through the first reactor or the second reactor with respect to time.
 2. The power conversion device according to claim 1, further comprising: a current sensor for measuring the value of the current flowing through the first reactor, wherein the control unit determines whether or not the first load current value is within the first current value range on the basis of the pattern of change in the value of the current flowing through the first reactor measured by the current sensor with respect to time during the first control, and wherein the control unit determines whether or not the second load current value is within the second current value range using the value of the current flowing through the first reactor measured by the current sensor as the second load current value during the second control.
 3. The power conversion device according to claim 1, further comprising: a current sensor for measuring the value of the current flowing through the second reactor, wherein the control unit determines whether or not the second load current value is within the second current value range on the basis of the pattern of change in the value of the current flowing through the second reactor measured by the current sensor with respect to time during the second control, and wherein the control unit determines whether or not the first load current value is within the first current value range using the value of the current flowing through the second reactor measured by the current sensor as the first load current value during the first control. 